This invention relates generally to methods and apparatus for detecting and locating wear of a conduit carrying a flowing substance and more particularly, but not by way of limitation, to methods and apparatus for detecting and locating the loss of metal from the interior surfaces of production equipment and flow lines used for transporting hydrocarbons and entrained particulate substances in a hydrocarbon production system.
Substances flowing through conduits (conduits being defined to include fluid-carrying elements such as may be found, for example, in hydrocarbon production equipment including valves, pumps, tubing, processing equipment, pipelines and the like) tend to wear the interior surfaces of the conduits. In particular, erosion occurs in an oil production conduit through which hydrocarbons having sand and other particulate substances entrained therein flow from a well. Erosion of the conduit occurs when the entrained particles impact on the interior surface of the conduit. Erosion can also be caused by cavitation which is the process where pressure differences cause bubbles to form in the flowing substance and to collapse at the interface between the flowing substance and a pipeline, for example, thereby producing wear on the pipeline. Erosion especially occurs within the pipeline on the outside of a turn just downstream of a change in direction of the flow or where there is a large change in pressure.
It is important to detect this erosion because if the erosion is allowed to continue, the conduit can fail when the wall thickness is reduced to the extent that the wall thickness can no longer withstand the pressure exerted by the flowing substance. Conduit failure can adversely affect the environment and the hydrocarbon production system. More generally, these wear processes can similarly affect an overall hydrocarbon production system and the production equipment contained therein.
Several types of devices contemplated to be used to determine the particulate content of a flowing fluid have been proposed. By monitoring the particulate (e.g., sand) content of the flow, these devices are proposed to be able to determine the amount of erosion caused by particulate impingement. Such devices have proposed the use of flow sampling and filtering techniques and the utilization of erodable sensors. However, very few devices can detect cavitation erosion.
One of the devices for monitoring erosion is disclosed in U.S. Pat. No. 3,906,780 to Baldwin. This patent discloses a means for continuously monitoring a fluid stream in a conduit to indicate the presence or increased presence of particulate material, such as sand, in the fluid stream. The continouos monitoring is disclosed to occur by detecting in the conduit vibrations, or "pinges," caused by the kinetic energy given up by the fluid stream as it impinges on the inner surface of the conduit and by generating electrical signals having frequencies representative of the vibrations and, thus, the kinetic energy. The Baldwin patent discloses that these vibrations have frequencies in excess of 100 kilohertz. It appears from the Baldwin patent that higher frequencies of the order of 700 kilohertz are preferable to detect because the Baldwin patent mentions that background noise up to 250 kilohertz has been detected without indicating the kinetic energy given up by the sand impinging upon the interior of the conduit. In other printed documents pertaining to a sand detector device related to the device described in the Baldwin patent, it is stated that the desirable frequencies to monitor are within the range between 650 kilohertz and 750 kilohertz. See, Mullins, Baldwin and Berry, Surface Flowline Sand Detection, Society of Petroleum Engineers (SPE) Paper No. 5152 (1974); Foster and Linville, A Method of Monitoring Sand Production in a Flowing Well Stream, SPE Paper No. 8214 (1979); and material from Oceanography International Corporation pertaining to the Sonic Sand Detector [entitled Sonic Sand Detector (1974), An Evaluation Report--The Sonic Sand Detector (1974), and Flowline Sand Detector/Monitor (1978)]. Related disclosures are set forth in U.S. Pat. No. 3,816,773 to Baldwin et al. and U.S. Pat. No. 3,841,144 to Baldwin.
Other types of apparatus and methods are disclosed in U.S. Pat. No. 3,384,181 to Maly et al., U.S. Pat. No. 3,854,323 to Hearn et al., U.S. Pat. No. 3,908,454 to Mullins, in Stuivenwold and Mast, New Instrumentation for Managing Sand-Problem Prone Fields, SPE Paper No. 9368 (1980), and in Swan and Reimer, Sand Probes Trim Equipment Erosion, Oil & Gas J. 45 (May 27, 1974). Sound responsive devices and methods proposed to be used in investigating blast effects are disclosed in U.S. Pat. No. 3,509,764 to Baldwin et al. and U.S. Pat. No. 3,563,311 to Stein.
In addition to merely detecting that wear is occurring in a conduit, it is important to locate where in the conduit the wear is occurring. This is important because, for example, if the wear is occurring at a specific location, failure may occur relatively quickly, whereas if the wear is only occurring continuously along the length of the conduit, then failure may not occur as quickly.
Although various methods and devcies have been proposed, such methods and devices have several shortcomings. For example, these devices generally lack the desirable sensitivity by which the erosion can be accurately determined. They also cannot locate precisely the area where wear is occurring. Additionally, there is generally no means for providing instantaneous readings of the erosion damage. Furthermore, several of the proposed systems require that equipment be inserted into the flow stream. This requires shutting down production while the equipment is installed or modified. The insertion of equipment into the flow stream also increases the turbulence which increases the erosion thereby further shortening the life of the conduit. Additionally, the inserted equipment, such as probes, are generally eventually destroyed by erosion; thus the life of the inserted probe is limited. These devices also generally have the shortcoming of being difficult to calibrate because they detect the sand content of the flow stream rather than the actual metal loss of the conduit. Because metal loss does not always occur merely because there is sand present, it is difficult to accurately correlate the sand content to the actual erosion damage.
Therefore, there is the need for an apparatus and a method for determining a direct indication of the actual erosion, and the location thereof, resulting from a particulate substance impacting on or from a cavitation process acting on the interior of the conduit. Such apparatus and method should provide accurate, reliable and instantaneous detection of the actual extent of erosion. The apparatus should also be attachable to existing systems without having to shut down the systems.